The present invention relates to the fabrication of optical fiber and optical fiber preforms.
While potentially useful in a wide variety of applications, the present invention evolved and was further developed in the field of optical fiber manufacture. Optical fibers are thin strands of glass capable of transmitting a light wave signal containing a large amount of information over long distances with very low loss. An optical fiber typically consists of an inner cylinder made of glass, often referred to as the core, surrounded by a cylindrical shell of glass or plastic of lower refractive index, often referred to as the cladding.
Optical fibers have traditionally been manufactured by first constructing a preform of appropriate composition and then drawing fiber from that preform. A typical preform generally assumes the form of a solid, concentric glass rod having a length of about one meter and a typical diameter of 10-100 mm. The core of this preform is a high purity, low loss glass such as germanium silicate glass having a diameter of about 1-40 mm. The cladding is a layer of glass which surrounds the core and which has a lower index of refraction than the core.
There are a number of fabrication processes in use today to manufacture such a preform. In one process, commonly known as the lateral soot deposition technique and described in U.S. Pat. Nos. 3,711,262 and 3,876,560, glass particulate matter and doped halides are formed in a hydrolysis burner and deposited on a starting member such as a glass rod. Additional layers of glass, including a cladding layer, are deposited on the rod and the combination is consolidated onto a transparent rod by heating in an inert environment. This process, requires many passes (up to 200) of the hot soot stream and is therefore costly and time consuming. In addition, after the soot is deposited, the preform must be sintered in a controlled inert atmosphere such as helium, which is also very costly. Moreover, these additional requirements require extensive process controls that can even further delay production and increase costs.
Another fabrication process is commonly referred to as the modified chemical vapor deposition (MCVD) technique. In this technique, glass precursor vapors are directed through a hollow glass cylinder which is heated sufficiently to start a homogeneous reaction within the glass cylinder. During this reaction, glass particulate matter is formed, deposited on the inside of the glass cylinder, and subsequently fused into the cylinder by traversing the heat source. This technique also has problems related to inefficient deposition rates and starting tube needs which, in turn, negatively impact manufacturing economics and production schedules.
Still another technique for the fabrication of fiber preforms is the vapor.axial deposition process, or more commonly VAD. This process, described in U.S. Pat. No. 4,062,665, involves simultaneous flame deposition of both core and cladding soots onto the end of a rotating fused silica-bait rod. As the porous soot preform grows, it is slowly drawn through a graphite resistance furnace (carbon heater) where it is consolidated into a transparent glass preform by zone sintering. This process has all the disadvantages and problems associated with a flame hydrolysis burner containing doped halides, as found in the lateral soot deposition technique described above, except in this case there are two hydrolysis burners to control. The process control of the finished preform and the control of both burners must be precise.
In still another method of manufacturing an optical fiber preform, the core is manufactured from an inner solid doped silica glass rod and one or more sleeving tubes. In this method, as described in U.S. Pat. Nos. 4,154,591 and 4,596,589, a core rod is placed within a sleeving tube. The tube is then collapsed onto the rod by slowly traversing a heat source over the entire length of the tube. British patent Application GB 22284206 suggests welding a supporting rod (with a sealing-up part for sealing the sleeving, or over-cladding, tube) to a core rod and a supporting tube to the sleeving tube, said supporting tube having a purity different than that of the sleeving tube and including a ring for centering the core rod. The tube is then collapsed onto the rod by slowly traversing a heat source over the entire length of the tube, while rotating the assembly on a lathe. The methods disclosed in the above patents result however in slow and expensive processes in that the tube and rod are completely collapsed into a solid multilayered cylindrical mass prior to the actual drawing of the fiber.
An alternative method for collapsing a sleeving tube onto a glass rod is disclosed in Japanese patent application with publication no. 63-170235. In said patent it is suggested to collapse a first end of the tube onto the rod, then applying a negative pressure to the inside of the tube and eventually collapsing the opposite end of the tube onto the rod. Japanese Patent Application JP 60-155542 discloses a method wherein a core rod and a sleeving tube are disposed into a heating furnace for drawing and the respective bottom ends are softened by heat, fused, joined, and drawn downward to mold a fiber.
According to what observed by the Applicant, when the tube is collapsed onto the rod, particular attention should be paid in not introducing asymmetries into the preform geometry during both the fabrication and/or collapse of the preform into a solid mass, which asymmetries may be reflected in the cross-section of the resulting fiber, with consequent negative impact on the transmission properties of the fiber. In particular, when manufacturing a simple two-layered preform by collapsing a tube onto a rod, attention should be paid in correctly aligning the tube and the rod at the beginning of the process and maintaining the tube centered onto the rod during the whole collapsing process, for avoiding such asymmetries. At this regard, the Applicant has noticed that, according to the prior art methods, the alignment of the tube with the inner rod is achieved directly on the lathe for carrying out the collapsing of the tube onto the rod. This operation is however particularly troublesome, as the lathe is generally in a vertical position and the correct alignment of the tube with the rod generally requires specific glass working skills to be realized. In addition, the applicant has observed that also the temperature of the heat source used for collapsing the tube should be accurately controlled along the entire collapsing process. In particular, when only the opposite ends of the tube are collapsed onto the respective ends or the rod, particular attention should be paid to the heating of the uncollapsed zone of the tube, in order to minimize thermal stress areas in the assembled preform, while avoiding undesired collapsing in this zone.
Accordingly, the present invention is directed to a method of and an apparatus for manufacturing optical fibers by utilizing an improved direct sleeving technology that substantially obviates one or more of the problems observed by the Applicant associated with the prior art methods.
Additional features, objectives and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the method and apparatus particularly pointed out in the written description and claims hereof as well as the appended drawings.
To attain these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, the invention provides a fiber optic preform and method for making the same by centrally positioning a core rod within a sleeving tube, thereby providing an annular gap between the outer surface of the rod and the inner surface of the tube, thermally collapsing the extreme ends of the tube onto the respective extreme ends of the rod while maintaining the annular gap uncollapsed over a major length of the tube. A force for biasing the sleeving tube inwardly toward the rod, e.g. vacuum, is preferably applied to said annular gap so to facilitate the collapsing of the tube onto the rod.
The completed preform is then inserted into a drawing tower where heat is applied to one end of the preform so that the uncollapsed part of the sleeving tube collapses onto the rod as the fiber is being drawn from the extreme of that same collapsed end.
In particular, an aspect of the present invention relates to a method of making an optical fiber preform by inserting a glass rod into a glass tube, thereby providing an annular gap between the outer surface of the rod and the inner surface of the tube, and thermally collapsing the extreme ends of the tube onto the respective extreme ends of the rod while maintaining an uncollapsed annular gap over a major length of the tube, comprising the steps of:
centrally positioning a rod within a tube, providing an annular gap between the outer surface of the rod and the inner surface of the tube;
forming a mechanical seal between one end of said tube and said rod, while aligning said tube with said rod with respect to their longitudinal axis;
thermally collapsing a first section of the tube onto the rod by traversing a heat source along said first section of the tube at a predetermined collapse speed, said section being located at the opposite unsealed end of said tube, in order to sealingly close said opposite end of said tube and said rod;
applying a force for biasing the tube inwardly towards the rod;
moving the heat source towards a second section of the tube, located in the proximity of the sealed end of the tube, by traversing said heat source at a predetermined traverse speed, so to avoid any collapsing of the tube onto the rod between the two sections and to avoid any thermal cracking of the preform;
thermally collapsing said second section of the tube onto the rod by traversing said heat source along said second section of the tube at substantially said predetermined collapse speed.
Preferably, the force for biasing the tube inwardly towards the rod is achieved by applying a vacuum to the annular gap through the mechanical seal.
According to a preferred aspect of the present invention, said predetermined traverse speed of the heat source is from about two to about eight times higher than said predetermined collapse speed.
According to a preferred aspect of the present invention, the above method comprises:
centrally positioning the rod within the tube and forming the mechanical seal and alignment as above;
thermally collapsing a first portion of said first section of the tube, by traversing the heat source along said first portion of the tube at a predetermined collapse speed, in order to sealingly close said first portion of said tube onto said rod;
applying the vacuum;
moving the heat source to the second section of the tube and back to the first portion, traversing it at a first predetermined traverse speed;
thermally collapsing the remaining portion of said first section of the tube, by traversing the heat source along said remaining portion at substantially said predetermined collapse speed;
moving the heat source to the second section of the tube, traversing it at a second predetermined traverse speed;
thermally collapsing the second section of the tube onto the rod by traversing said heat source along said second section of the tube at substantially said predetermined collapse speed.
According to a preferred aspect, said first portion of the first section being collapsed is from about 10% to about 30% of the total section of the tube to be collapsed.
Said first and second predetermined traverse speed may be both from about two to about eight times higher than said predetermined collapse speed or, preferably, said first predetermined traverse speed is from about two to six times higher than said predetermined collapse speed, while said second predetermined traverse speed is from about four to eight times higher than said predetermined collapse speed.
According to a further preferred aspect, the step of forming a mechanical seal between one end of said tube and, said rod while aligning the rod within the tube is carried out by using a device, thus forming an assembly comprising the device and the aligned rod and tube, said assembly being then mounted on a glass lathe.
According to another aspect of the present invention, the device for aligning the rod within the tube and forming a mechanical seal between one end of said tube and said rod comprises a threaded body (37), threaded rings (38, 39 and 40), O-rings (12, 13 and 14) and split rings (32 and 33), the O-rings 12, 14 and the split rings 32, 33 cooperating, upon rotation of the threaded rings 38, 40, to provide the centralization of the rod with the tube and the mechanical seal.
According to a preferred aspect, said split rings are opened, being provided with a gap along their circumference.
A primary result of the new sleeving technique embodied in this invention is that, in contrast with conventional sleeving; methods, the entire length of the tube/cladding is not collapsed onto the core rod until the drawing phase of the manufacturing process. Because the actual sleeving is accomplished during fiber drawing, the invention greatly reduces preform and fiber production times and costs. The invention also offers a better control over the alignment of the assembled preform and on fiber geometry, as well as a reduction of the thermal stress areas in the assembled preform.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.